update SMS docs to include GitLab

[khorne3]

Changes:

• Updated the quickstart
• Updated the information in Semgrep Managed Scanning, including splitting out GitHub- and GitLab-specific information into their own pages.

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Previews:

• Quickstart for Semgrep Managed Scans
• Semgrep Managed Scans
  • GitHub
  • GitLab

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Please ensure

• A subject matter expert (SME) reviews the content
• A technical writer reviews the content or PR
• This change has no security implications or else you have pinged the security team
• Redirects are added if the PR changes page URLs
• If you have changed any header tag links (doc/#this-kind-of-anchor), update all instances of that link

[netlify]

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[s-santillan]

Minor stuff only, thank you for piecing out all the information!

[Zhou-Duo]

thank you katie 🎉!! these changes are very comprehensive! I've left some comments, but could you hold off on further work for now as the team's currently discussing whether we should release this private beta doc or keep it internal until the public beta (which we expect to launch in two weeks). I'll get back to you asap by EOD, in the worst case, we'll just frame it as 'public beta' and release it two weeks later. sorry for any inconvenience this may cause T T